What are the requirements for a radiation survey according to AMS 2750F (Aerospace Material Specifications)?
First of all, it is important to state that such a survey is required under AMS 2750, exclusively for furnaces used for the Thermal Treatment of Aluminium Alloys and it is limited to process temperatures above 427°C.
Therefore, it is mainly for plants dedicated to Thermal Treatment of Solubilisation. The solubilisation temperature for many alloys is very close to the eutectic melting point and this is why in these types of furnaces, it is also necessary to test the radiation effect directly on the metal as well as measuring and certifying the temperature uniformity of the operating volume qualified within the limits required by the applicable specifications.
The survey must be carried out to check the radiation from the heating sources, whether they are electric resistors or gas-heated radiating pipes, positioned on the walls or on the floor or on the ceiling of the furnace.
Therefore, it is necessary first of all, together with the furnace designer, to identify where in the furnace the heating elements are distributed.
Once the walls with heating elements have been identified, it is necessary to calculate the area of each of the walls in order to count the number of panels required which need to be distributed evenly during the test; one panel every 0.93 m2.
A PRACTICAL EXAMPLE
Example: if the furnace has a cube shape whose sides measure 1.5 m and the heating elements are positioned on 4 walls (e.g. ceiling, floor and two side walls) it will be necessary to evenly distribute 2 panels on each wall; therefore 8 panels in total. The panels must be positioned on the walls parallel to the heat source and corresponding to the walls of the loading drum representing the qualified work zone - the same drum used for the “TUS” - Temperature Uniformity Survey.
The survey must be performed at initial qualification and be repeated in the event of plant modifications which may have an impact on the radiation from the heating source. The survey must be carried out at the maximum operating temperature used in production.
FEATURES OF THE PANELS
The square panels must be made of aluminium 6061, with sides measuring 300 mm (approx.) and a thickness of less than 3.1 mm. The panels must be subjected, before the radiation survey, to thermal treatment within the range of 520 to 545 °C ± 6.0 °C, for a period of time that is suited to the thickness of the panels used. The thermal treatment must be performed and then cooled in the air.
Each panel must be fitted with a temperature sensor to be positioned in the centre of the panel. Particular care must be taken to ensure that the hot junction of the temperature sensor is in contact with the panel material.
The panels must be positioned and distributed on the walls of the drum parallel to the heat sources and the survey must be performed by using the same procedure and acquisition required for the “TUS” - Temperature Uniformity Survey.
Throughout the survey, the temperature of all the sensors must be within the required temperature uniformity (for example, ± 6°C for class 2 ovens).
If one or more sensors exceeds this limit, then the survey has failed and the oven cannot be used for production; the cause must be removed and a new test must be performed.
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